Generally speaking, the present invention relates to the field of fine measurement; when properly handled, it enables force to be measured independently of position, especially by means of a so-called tonometer, whereby the gravitational forces that are present during every measurement process having no bearing on the results or can, at least, be taken into consideration through appropriate measures in the hardware or software.
To measure the intraocular pressure of patients, it is customary practice, following a general underlying principle, to press a suitable plane member against the eyeball, which is assumed to be of spherical configuration, and to measure the pressure which must be exerted to produce a given diameter of the circular area aplanated thereby when this given diameter is attained.
In a known device, a lever which can be pivoted about a stationary fulcrum is employed in this connection; this lever performs an arc-based pivotal motion toward the eye beneath a forehead support (to achieve a fixed position) and can therefore be pressed against the eye with a measurable exertion of force. At the opposite end of the lever on the other side of the fulcrum, an electric motor engages via a comparatively soft spring, thus implementing the advancement toward the eye that is required for performance of the measurement.
In a known measurement apparatus of this type, suspension of the pendulum-like lever can be problematic, as it describes an arc as a result of its fulcrum mounting. Nor is the possibility able to be excluded that measurement errors could occur as a result of the lack of dynamic balancing. Moreover, miniaturization efforts are quickly exhausted with pendulum apparatuses of this type.
Moreover, a tonometer has already been proposed (in unprepublished German Patent Disclosure Document No. 3,818,434) in which a first measurement slide and a second counter-slide are disposed on both sides of bearing rollers, the slides performing a mechanically coupled opposite motion one relative to the other, whereby the effect of the measurement slide and the counter-slide pressing against the bearing rollers is produced by an alternating magnetic effect exerted one upon the other. The two slides can therefore perform a free, opposite displacement motion, without being hindered by the respectively acting weight. The measurement slide then mounts a sensor head in the form of a prism, whose reflection properties are altered through the aplanation of the eyeball when pressure is exerted, and are identified by a photosensitive element, and are exploited for producing a signal when a given aplanation diameter is attained.
To this extent, all previously known tonometer systems are based upon the principle of performing an individual measurement, whereby it is agreed that this individual measurement is performed when an aplanation diameter of exactly 3.06 mm has been attained; empirical studies have shown this value to be particularly suitable, especially since various influencing factors more or less mutually offset one another in a given range, for example adhesion force and diaphragm stiffness (paper by Goldmann and Schmitt in DE-Ophthamalogica, 134, 1957).
In addition, it must also be taken into consideration that the best possible decoupling of the parameters of tear fluid volume and aplanated area can be achieved in such measurements, whereby it has also already been proposed that a so-called zero aplanation be performed in order to determine a volume of tear fluid which is then included in the determination of the area by means of a compensation calculation. However this always involves the necessity of performing individual measurements (such as for zero aplanation and for the above-mentioned diameter of 3.06 mm), which can involve serious errors merely as a result of the effect of random errors, whereby incorrect behavior on the part of the examinee at the moment the measurement is performed can also seriously falsify the measurement results.